(1) Field of the Invention
The present invention relates generally to fiber optic sensor systems and, more specifically, to apparatus and method for fiber optic sensing including generating, propagating, and detecting highly repeatable pulses through a fiber optic sensor system.
(2) Description of the Prior Art
Prior art fiber optic sensor systems, such as fiber optic acoustic sensor systems, typically incorporate either Michelson or Mach-Zehnder type interferometers. These interferometers have high sensitivity and high dynamic range. In using these interferometers, a continuous light wave is split into two paths. The first path is used as a reference giving rise to a reference beam, while the second path is used as the sensor path which provides the signal beam. The acoustic field modulates the lightwave in the second path, usually by means of a fiber optic transducer. The two beams (reference beam and the signal beam) are recombined and the resulting interference is detected at a photoreceiver. Noise from extraneous acoustic fields or vibration may be picked up by the reference beam or by the lead cable of the fiber optic sensors. Phase noise of a coherent light source, as detected by the photoreceiver, increases with increased interferometer path differences. The response of a Mach-Zehnder interferometer may be limited by phase noise if the path differences for the reference and the signal beams are large. As well, large phase drifts may be produced in such systems due to temperature effects on the interferometer. For a homodyne system, a phase tracking scheme is needed to compensate for the large phase drifts that result from temperature effects on the interferometer. Amplitude noise may also be produced by self-interference of the lightwave due to scattering or reflecting within the fiber optic paths.
Previous methods for generating light pulses for general usage are not always suitable for fiber optic sensor systems. For instance, a previous pulse generating method as discussed in more detail in the references listed below, chirps the light source itself. For purposes of fiber optic sensor system, this method is considered to be difficult to control and produces noisy signals that are not highly repeatable. Another previous method, as also discussed below, uses the non-linear properties of optical fiber to chirp a pulse but requires high light power to generate these signals.
U.S. Pat. No. 4,486,657, issued Dec. 4, 1984, to I. J. Bush, discloses a fiber optic acoustic sensing system for tracking a phase shift linearly over a wide range. Light from a laser is split and coupled into both legs of a fiber interferometer. One leg is phase modulated by the acoustic signal while the other leg is phase modulated by first and second piezoelectric cylinder modulators. The light signals in the two legs are combined to produce an error signal. The error signal is fed back to control the first modulator.
U.S. Pat. No. 4,588,957, issued May 13, 1986, to Balant et al., discloses a system wherein an optical pulse is passed through a nonlinear dispersive delay line, which chirps the pulse by the nonlinear process of self-phase modulation and simultaneously and interactively broadens the pulse by the process of group velocity dispersion. By making the optical pulse suitably intense, a single mode optical fiber may act as the nonlinear dispersive delay line.
U.S. Pat. No. 5,430,569, issued Jul. 4, 1995, to Blauvelt et al., discloses a chirp signal generator coupled to the signal path of an RF input signal that carries information to modulate the laser optical output. In some cases, the frequency of the chirp generating signal may result in second order intermodulation products falling within the information band. In such cases, the RF input signal is predistorted to offset the expected distortion products.
U.S. Pat. No. 5,453,868, issued Sep. 26, 1995, to Blauvelt et al. is a continuation of the above patent, i.e., U.S. Pat. No. 5,430,569, to the same inventors.
U.S. Pat. No. 4,313,185, issued Jan. 26, 1982, to J. L. Chovan, discloses an acoustic vibration sensing system having principal application to hydrophones and operating under the optical heterodyning principle. The sensor employs a pair of single mode fibers, optically coupled by a path whose length is varied by the acoustic vibrations. The path includes a partially reflecting discontinuity at the sensitive end of each fiber. Optical signals of one frequency are supplied to one fiber, and of another frequency to the other fiber. Optical signals of the same difference frequency emerge from the xe2x80x9cdry endxe2x80x9d of each fiber. When these two emergent signals are photodetected and the phase or frequency difference is obtained, the acoustic vibration is sensed.
U.S. Pat. No. 4,363,114, issued Dec. 7, 1982, to Bucaro et al., discloses an optical system for frequency modulation heterodyne detection of an acoustic pressure wave signal. An optical beam is directed into a Bragg cell outside of the fluid medium in which acoustic signals are to be detected. The Bragg cell modulates the incident beam such that two beams of different frequency exit the cell. The two beams are combined and transmitted to a fiber optic transducer disposed in the medium. The transducer includes two coiled optical fibers, a reference fiber and a signal fiber, each of which has a different sensitivity to incident acoustic pressure wave signals. A filter in the signal fiber transmits on a fraction of the light. The two parts of the split beam exiting the coiled optical fibers are coupled into another optical fiber and transmitted to a photodetector from which the output signal is processed to indicate the detection of an acoustic pressure wave signal.
In summary, while the prior art shows various specific fiber optic acoustic systems and means for generally producing optical pulses, the above disclosed prior art does not show an acoustic sensor system using a pulsed system for fiber optic acoustic signal detection. The prior art does not disclose a pulsed fiber optic sensor system wherein pulses may pass through several fiber optic sensor arrays. Consequently, there remains a need for a system that uses optical pulses in an optical sensor system to reduce coherent light phase noise, that provides improved optical pulses suitable for low noise optical sensors, and that eliminates the need for a phase tracker circuit. Those skilled in the art will appreciate the present invention that addresses the above and other problems.
Accordingly, it is an object of the present invention to provide an improved fiber optic sensor system.
It is yet another object of the present invention to use heterodyne dual light pulses to interrogate the fiber optic transducers.
It is another object of the present invention to reduce the noise associated with the pulses by chirping the dual pulses.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims.
In accordance with the present invention, a method for a fiber optic sensor system for sensing a physical phenomena is disclosed. The method includes steps such as generating first and second light pulses from a coherent light source such that the second light pulse is time delayed with respect to the first light pulse. The first and second light pulses are directed into a lead fiber optic cable connected to one or more fiber optic transducers. The first and second light pulses are then directed from the one or more fiber optic transducers for photo detection to produce an electrical signal containing information related to the physical phenomena.
In one embodiment, a single light pulse is initially produced from the coherent light source, such as by gating an optical amplitude modulator as discussed below, although the optical amplitude modulator could be used to produce both pulses. That pulse travels on a fiber optic path into a splitter to form a first fiber optic light path and a second fiber optic light path. The first light pulse is produced on the first fiber optic path and the second light pulse is produced on the second fiber optic path. The second light pulse is delayed with respect to the first light pulse. The first pulse is preferably frequency shifted or frequency modulated. The first and second light pulses are then recombined in a single optical path.
In one preferred embodiment, the single light pulse from the coherent light source is chirped by some means such as with a surface acoustic wave device. A unique method for chirping an optical pulse is provided that includes steps such as producing an electrical pulse signal and applying the electrical pulse signal to a surface acoustic wave device to produce a chirp signal. The chirp signal is applied to an optical frequency modulator to chirp the optical pulse to produce a chirped optical pulse. The optical fiber on which the chirped optical pulse travels is split to produce first and second chirped optical pulses.
After being propagated through the optic fiber sensor system, the first and second light pulses are reflected from the fiber optic transducers through a lead fiber optic cable, as discussed below. A circulator is preferably used both for directing the first and second light pulses into the lead fiber optic cable and for directing the first and second light pulses from the lead fiber optic cable for photo detection.
The pulsed system for use with the fiber optic sensor array includes elements such as a coherent light source and means for producing the first and second light pulses from the coherent light source such that the second light pulse is delayed with respect to the first light pulse. A photodetector receives the first and second light pulses after propagation through the one or more fiber optic transducers to produce an electrical signal containing information related to at least one physical phenomena. A delay line may be used in series with the second fiber optic path to delay the second pulse. A frequency shifter in series with the first fiber optic path is used to shift the frequency of the first pulse to a carrier frequency. A circulator may be used for directing the first and second light pulses between the means for producing the first and second light pulses, the one or more fiber optic transducers, and the detector.